EP0580312A1 - Catalyst and process for removing nitrogen oxides - Google Patents

Abstract

Disclosed herein are an inexpensive catalyst for removing nitrogen oxides effectively from an exhaust gas by either catalytic reduction or catalytic decomposition, and a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst. The invention will eliminate the disadvantages -- high material cost and complex process for catalyst preparation -- involved in the conventional technology. The catalyst contains as the major constituent a multiple oxide of the CaO-Fe_xO type containing 5-50 wt% CaO, said catalyst reducing or decomposing nitrogen oxides. The process is designed for denitration of nitrogen oxides by the aid of said catalyst.

Description

• The present invention relates to a catalyst for removing nitrogen oxides from a combustion exhaust gas (such as from a sintering machine). More particularly, the present invention relates to a catalyst for removing nitrogen oxides from a combustion exhaust gas by reduction or decomposition. (The catalyst promotes reduction by a reducing agent which is originally present in an exhaust gas or is added to an exhaust gas afterward. The catalyst also performs catalytic decomposition by itself.) The present invention also relates to a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst.
• One of the most important problems associated with air pollution is the removal of nitrogen oxides.
• There are several means to remove nitrogen oxides from exhaust gases. They are broadly classified as (1) denitration by catalytic reduction, (2). denitration by catalytic decomposition, (3) denitration without catalysts, and (4) denitration by electron beam irradiation. The first two are generally preferred because of low operation cost and high performance.
• Exhaust gas from a sintering machine needs the reduction of nitrogen oxides. This object is usually achieved by denitration with a special equipment attached to the sintering machine. Other means to this end include "exhaust gas recycling" to control fuel combustion (as disclosed in Japanese Patent Laid-open No. 70008/1978) and "multi-stage charging" (as disclosed in Japanese Patent Laid-open No. 80202/1979). These technologies have the disadvantage of requiring additional equipment for denitration, exhaust gas recycling, or multi-stage charging. On the other hand, an attempt has been made to carry out denitration by mixing an exhaust gas with a substance, such as an ammonium compound, as disclosed in Japanese Patent Laid-open No. 82304/1979. This attempt, however, is not successful because of the possibility of ammonia leaking from the system to cause another environmental pollution and the difficulties of process control. Now, the "Scale charging process" as disclosed in Japanese Patent Laid-open No. 53704/1977 is attracting attention from the standpoint of environmental safety and high denitrating performance. To improve this technology, the present inventors paid their attention to denitration with a catalyst.
• The conventional catalysts for denitration are based mostly on noble metals, and hence they are expensive per se and need to be supported on a carrier by a complex process. Therefore, they are not generally used in the field of air pollution control.
• In view of the foregoing, the present invention was completed to eliminate the disadvantages -- high material cost and complex process for catalyst preparation -- involved in the conventional technology. It is an object of the present invention to provide an inexpensive catalyst for removing nitrogen oxides effectively from an exhaust gas by either catalytic reduction or catalytic decomposition. It is another object of the present invention to provide a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst.
• Fig. 1 is a schematic representation of the experimental apparatus used in the present invention.
• Fig. 2 is a graph showing the relationship between the ratio of denitration and the concentration of CaO (in catalytic reduction).
• Fig. 3 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic reduction).
• Fig. 4 is a graph showing the relationship between the ratio of denitration and the concentration or CaO (in catalytic decomposition).
• Fig. 5 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic decomposition).
• Fig. 6 is a graph showing the relationship between the value of x in Fe_xO and the ratio of denitration (in catalytic reduction).
• Fig. 7 is a graph showing the relationship between the value of x in Fe_xO and the ratio of denitration (in catalytic decomposition).
• Fig. 8 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic reduction).
• Fig. 9 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic decomposition).
• Fig. 10 is a schematic representation of the experimental apparatus simulating a sintering machine.
• Fig. 11 is a graph showing the effect of denitration in the sintering machine model.
• According to the present invention, the catalyst for removing nitrogen oxides should be composed mainly of a multiple oxide of the CaO-Fe_xO type containing 5-50 wt% CaO. It promotes the removal of nitrogen oxides from an exhaust gas by reduction with a reducing agent which originally exists therein or is added thereto afterward. It also removes nitrogen oxides from an exhaust gas by decomposition. According to the present invention, the removal of nitrogen oxides from an exhaust gas is accomplished by reduction or decomposition by the aid of said catalyst.
• According to the present invention, the catalyst should be composed mainly of a multiple oxide of the CaO-Fe_xO type containing 5-50 wt% CaO as an essential ingredient. With a CaO content lower or higher than specified above, the catalyst will be not effective. Fe_xO in the multiple oxide CaO-Fe_xO may be in any form of FeO, Fe₃O₄ (FeO·Fe₂O₃), or Fe₂O₃; however, it should preferably have an atomic ratio of Fe/O in the range of 0.67 to 1.00.
• The functions and effects of the present invention will be described with reference to the following experimental results.
• Fig. 1 is a schematic representation of the apparatus used for experiments. There is shown a nitrogen oxides-containing gas generator (1), in which nitrogen oxides, carrier gas, optional reducing gas, etc. are mixed in a prescribed ratio. The thus prepared nitrogen oxides-containing gas passes through a conduit tube (2) to enter a sealed tube (crucible) (3). The gas jets out from the nozzle (4) and comes into contact with a catalyst (6) heated to a prescribed temperature by a heater (5). The temperature of the catalyst is monitored by a thermocouple (7). Where it is necessary to test the catalyst for ability to promote reduction, the nitrogen oxides-containing gas should be incorporated with a reducing agent such as ammonia, carbon monoxide, and RX gas. After reduction or decomposition by the catalyst (6), the resulting gas is discharged through a conduit tube (9). It is partly caught by a gas sampling bag (8) and the remainder is tested for the concentration of nitrogen oxides by an analyzer (10). Tables 1 to 3 show the experimental conditions, the gas composition, and the catalyst composition, respectively. Table 1

  Crucible diameter	24 mm
  Gas flow rate	2 N L/min
  Nozzle diameter	4 mm
  Crucible material	99.9% Al₂O₃
  Calcium ferrite	10 g

  Table 2

  Type of reaction		Gas composition
  Catalytic reduction	●	360 ppm·NO-2.5% CO-7.5% CO₂-Ar
  	▲	360 ppm·NO-5.0% CO-5.0% CO₂-Ar
  	■	360 ppm·NO-7.5% CO-2.5% Co₂-Ar
  Catalutic decomposition		400 ppm·NO-Ar

  Table 3

  Sample	CaO content	Iron oxide	Value of x in FexO
  C-W	25%	FeO	1.00
  C-M	25%	Fe₃O₄	0.75
  C-H	25%	Fe₂O₃	0.67

• The catalyst used in the experiment was prepared by fusing CaO and an iron oxide specified in Table 3. It was used in the pore-free state.
• The experimental results are shown in Figs. 2 to 5. The ratio of denitration (EL%) was calculated as follows from the amount of NO (C_NO ⁱⁿ) charged and the amount of NO (C_NO ^out) discharged. $${\text{EL\% = [(C}}_{\text{NO}}{\text{}}^{\text{in}}{\text{- C}}_{\text{NO}}{\text{}}^{\text{out}}{\text{) ÷ C}}_{\text{NO}}{\text{}}^{\text{in}}\text{] x 100}$$

  
• Figs. 2 and 3 show the ratio of denitration by catalytic reduction with CaO-FeO as an example of the catalyst. It is noted from Fig. 2 that the high ratio of denitration is achieved when the CaO content is in the range of 5-50%, especially in the neighborhood of 25%. It is noted from Fig. 3 that the high ratio of denitration is achieved when the reaction temperature is higher than 800°C if the CO/CO₂ ratio is lower than 0.6. (A sufficiently high ratio of denitration is achieved even at 500°C if the CO/CO₂ ratio is higher than 0.6.) Incidentally, it is amazing that the catalyst remains effective even in its molten state at a high temperature.
• Figs. 4 and 5 show the ratio of denitration by catalytic decomposition with CaO-FeO as an example of the catalyst. It also varies depending on the CaO content and reaction temperature as mentioned above. In other words, it is noted from Fig. 4 that if the CaO content is 5-50 wt%, the ratio of denitration is high, but otherwise it is low. It is also noted from Fig. 5 that the high ratio of denitration is achieved when the reaction temperature is higher than 900°C.
• Experiments were carried out under the same conditions as shown in Fig. 1 and Tables 1 and 2 (at 1300°C) to see how the catalyst varies in denitrating capability depending on the degree of oxidation of the calcium ferrite. Figs. 6 and 7 show the results of catalytic reduction and catalytic decomposition, respectively. It is noted that the maximum ratio of denitration is achieved when x = 1 (FeO) in either cases. Although good results are obtained in catalytic reduction even though the value of x is low (Fe₂O₃) so long as the CO/CO₂ ratio is high, it is desirable that the value of x be in the range of 0.67-1.00 for catalytic reduction and in the higher range of 0.9-1.0 for catalytic decomposition. Incidentally, the ratio of denitration shown in Fig. 7 will be greatly improved if the reaction area is increased. For this reason, it is desirable from the standpoint of industrial production that the catalyst be formed in lumps of proper size from fine powder. The catalyst becomes more effective as the reaction temperature increases, and it is effective even when it is in molten state at about 1200°C and above although it has a reduced specific surface area.
• Experiments on denitration were carried out using iron oxide pellets as the catalyst containing a multiple oxide of the CaO-Fe_xO type containing ca. 15 wt% CaO. They were prepared by mixing finely divided iron ore with a CaO-containing mineral, making the mixture into pellets, and heating the pellets at about 1300°C. The experimental conditions are shown in Tables 4 and 5. Table 4

  Chemical composition	Fe	62 %
  	CaO	4.4 %
  	SiO₂	3.0 %
  	Al₂O₃	1.4 %
  Porosity		27.0 %
  Diameter		9.2-9.8 mm

  Table 5

  Gas composition	For catalytic reduction	360 ppm·NO-5% CO-5% CO₂-Ar
  	For catalytic decomposition	400 ppm·NO-Ar
  Gas flow rate		2 N L/min
  Weight of catalyst		140 g
  Material of crucible		99.9% Al₂O₃

• The results of catalytic reduction are shown in Fig. 8. It is noted that the catalyst is effective when it is above 500°C. The results of catalytic decomposition are shown in Fig. 9. It is noted that the catalyst is effective when it is above 850°C.
EXAMPLE
• Since CaO-Fe_xO is one of the major constituents of sintered products, it is expected that if it is fed to the bed along with the raw material for sintering, denitration will take place as soon as NO_x occurs, without any adverse effect on the sintered products.
• To simulate the combustion in a sintering machine, coke breeze (as the major fuel for a sintering machine) was burned together with CaO-Fe_xO and the amount of NO_x was measured, using an apparatus schematically shown in Fig. 10. The experimental conditions are shown in Tables 6 and 7. The experimental results are shown in Table 8 and Fig. 11. It is noted that CaO-Fe_xO is twice as effective in denitration as scale. Table 6

  Items	Description
  Amount of coke	0.5 g
  Diameter of coke	1-3 mm
  Diameter of catalyst	1-3 mm
  Diameter of reactor tube	50 mm
  Preheating temperature
  	800°C
  Gas flow rate	2 N L/min
  Diameter of crucible	12 mm
  Material of crucible	Mullite

  Table 7

  Sample	Composition
  CaO-FexO	25% CaO-75% FeO
  Scale	By-product from steel making

  Table 8

  Sample charged	Ratio of catalyst to coke (by weight)	Amount of NOx discharged (ml)	Undecomposed NOx in discharged gas (wt%)
  Coke (control)	-	2.77	-
  CaO-FexO	10	2.44	11.9
  	20	2.25	18.8
  Scale	10	2.60	6.1
  	20	2.43	12.3

• The present invention provides a catalyst which, on account of its constitution mentioned above, is economical materialwise and processwise and is effective in the removal of nitrogen oxides by either reduction or decomposition.

Claims (9)

1. A catalyst for removing nitrogen oxides which comprises as the major constituent a multiple oxide of the CaO-Fe_xO type containing 5-50 wt% CaO, said catalyst promoting the reduction of nitrogen oxides.
2. A catalyst for removing nitrogen oxides as defined in Claim 1, wherein the multiple oxide of the CaO-Fe_xO type has a value of x in the range of 0.67 to 1.00.
3. A process for removing nitrogen oxides which comprises treating a nitrogen oxides-containing gas with a reducing agent in the presence of the catalyst as defined in Claim 1 or 2.
4. A process for removing nitrogen oxides as defined in Claim 3, wherein the catalyst defined in Claim 1 or 2 is used under the condition that the reaction temperature is higher than 500°C if the CO/CO₂ ratio in NO_x-containing gas is greater than 0.6 and the reaction temperature is higher than 800°C if the CO/CO₂ ratio in NO_x-containing gas is smaller than 0.6.
5. A catalyst for removing nitrogen oxides which comprises as the major constituent a multiple oxide of the CaO-Fe_xO type containing 5-50 wt% CaO, said catalyst decomposing nitrogen oxides.
6. A catalyst for removing nitrogen oxides as defined in Claim 5, wherein the multiple oxide of the CaO-Fe_xO type has a value of x in the range of 0.9 to 1.0.
7. A process for removing nitrogen oxides which comprises treating a nitrogen oxides-containing gas with the catalyst as defined in Claim 5 or 6.
8. A process for removing nitrogen oxides as defined in Claim 7, wherein the catalyst defined in Claim 5 or 5 is used under the condition that the reaction temperature is higher than 850°C.
9. A catalyst for removing nitrogen oxides as defined in any one of Claims 1, 2, 5 and 6, which is used in molten state or in lumps of proper size formed by sintering from fine powder.

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